1. Field of the Invention
The present invention relates generally to surface hardening of metals, and particularly to a method of laser treating Ti-6Al-4V to form barrier surface compounds using gas-assisted laser nitriding.
2. Description of the Related Art
Titanium alloys are metallic materials which contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures), are light in weight, exhibit extraordinary corrosion resistance, and have the ability to withstand extreme temperatures. Although “commercially pure” titanium has acceptable mechanical properties and has been used for orthopedic and dental implants, for most applications titanium is alloyed with small amounts of aluminum and vanadium, typically 6% and 4%, respectively, by weight. This mixture has a solid solubility which varies dramatically with temperature, allowing it to undergo precipitation strengthening. This heat treatment process is carried out after the alloy has been worked into its final shape but before it is put to use, allowing much easier fabrication of a high-strength product.
The American Society for Testing and Materials (ASTM) classifies titanium alloys by numerical grades. “Grade 5”, also known as Ti-6Al-4V, is the most commonly used alloy. It has a chemical composition of 6% aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen, and the remainder titanium. Grade 5 is used extensively in the aerospace, medical, marine, and chemical processing industries. Ti-6Al-4V is significantly stronger than commercially pure titanium while having the same stiffness and thermal properties. Among its many advantages, it is heat treatable.
This grade also exhibits an excellent combination of strength, corrosion resistance, weld and fabricability. Generally, it is used in applications up to 400° C., and its properties are very similar to those of the 300 stainless steel series, particularly stainless steel 316.
Titanium dioxide dissolves in titanium alloys at high temperatures, and its formation is very energetic. These two factors mean that all titanium, except the most carefully purified, has a significant amount of dissolved oxygen, and so may be considered a Ti—O alloy. Oxide precipitates offer some strength, but are not very responsive to heat treatment and can substantially decrease the alloy's toughness. In order to protect a titanium alloy, the formation of surface barrier compounds is desirable. Thus, a method of laser treating Ti-6Al-4V to form surface compounds solving the aforementioned problems is desired.